A period of 19 years (235 lunar months), after which the new and full moons return to the same days of the year. It was the basis of the ancient Greek calendar and is still used for calculating movable feasts such as Easter.
- Under the Julian calendar, the phases of the moon and hence the dates of Easter repeated roughly every 19 years, a period known as the Metonic cycle after the Greek astronomer Meton.
- Astronomers call this 19-year period the Metonic cycle, after the mathematician Meton who lived in Athens in the fifth century B.C., although there is evidence that the Babylonians knew of the synchrony earlier.
- The date of Easter, which is based on the Jewish festival of Passover, is still calculated using the Metonic cycle (via the golden number).
Named after Metōn, an Athenian astronomer of the 5th century bc.
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